Morphology formation mechanism and fluorescence properties of nano-phosphor YPO4:Sm3+ excited by near-ultraviolet light

Article abstract of DOI:10.1016/j.jallcom.2019.153535

A series of YPO₄:Sm³⁺ phosphors were prepared by the hydrothermal method. The composition, structure, morphology and luminescence properties of the samples were characterized and analyzed by means of X-ray diffractometer (XRD), scanning electron microscopy (SEM and EDS) and fluorescence spectrophotometer (FL). The morphology formation mechanism and fluorescence temperature characteristic of nano-phosphor YPO₄:Sm³⁺ were mainly studied. The results show that the products are single tetragonal phosphor yttrium ore structure with nanosphere morphology. Morphology formation mechanism of YPO₄:Sm³⁺ is nucleation - dissolution - recrystallization - crystal growth. The strongest excitation spectrum appears at 404 nm, which belongs to the ⁶H_5/2 → ⁴F_7/2 transition of Sm³⁺. The strongest emission spectrum appears at 603 nm, which belongs to the ⁴G_5/2 → ⁶H_7/2 transition of Sm³⁺. The chromaticity coordinates show that the phosphors are red. The optimum doping concentration of activator Sm³⁺ in YPO₄ matrix is 2%. When the doping concentration of Sm³⁺ is higher than 2%, concentration quenching occurs. The mechanism of concentration quenching is electric dipole - electric dipole interaction. The critical distance for energy transfer between Sm³⁺ is 1.899 nm. Nano-phosphor YPO₄:2% Sm³⁺ has good thermal stability, and its fluorescence lifetime is 217.99 μs. The activation energy of thermal quenching is 0.2566 eV.

Full text of DOI:10.1016/j.jallcom.2019.153535

Journal of Alloys and Compounds 821 (2020) 153535
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Morphology formation mechanism and fluorescence properties of
nano-phosphor YPO4:Sm3þ excited by near-ultraviolet light
,
b
,
c
,
f
,
,
b,
c
,
f
,
,
d
,
,
c
,
, d, f
Jinxiu Wu a
*, Mei Li a
**, Huiling Jia c f, Zhaogang Liu a f, Hengjun Jia c
,
Zhongzhi Wang e
a College of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou, Inner Mongolia, 014010, PR China
b School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China
c Key Laboratory of Green Extraction and Efficient Utilization of Light Rare-Earth Resources, Ministry of Education, Baotou, Inner Mongolia, 014010, PR
China
d College of Mechanical Engineering, Inner Mongolia University of Sciences and Technology, Baotou, Inner Mongolia, 014010, PR China
e Baotu Research Institute of Rare Earths,Baotou, Inner Mongolia, 014030, PR China
f Inner Mongolia Key Laboratory of Rare Earth Hydrometallurgy and Light Rare Earth Application, Baotou, Inner Mongolia, 014010, PR China
a r t i c l e i n f o
a b s t r a c t
A series of YPO4:Sm3þ phosphors were prepared by the hydrothermal method. The composition,
structure, morphology and luminescence properties of the samples were characterized and analyzed by
means of X-ray diffractometer (XRD), scanning electron microscopy (SEM and EDS) and fluorescence
spectrophotometer (FL). The morphology formation mechanism and fluorescence temperature charac-
teristic of nano-phosphor YPO4:Sm3þ were mainly studied. The results show that the products are single
tetragonal phosphor yttrium ore structure with nanosphere morphology. Morphology formation
Article history:
Received 9 September 2019
Received in revised form
9 December 2019
Accepted 24 December 2019
Available online 25 December 2019
mechanism of YPO4:Sm3þ is nucleation - dissolution - recrystallization - crystal growth. The strongest
Keywords:
6
excitation spectrum appears at 404 nm, which belongs to the H5/2
/
4F7/2 transition of Sm3þ. The
YPO4:Sm3þ
strongest emission spectrum appears at 603 nm, which belongs to the 4G5/2
/ .
6H7/2 transition of Sm3þ
Hydrothermal method
Morphology formation mechanism
Luminescence
The chromaticity coordinates show that the phosphors are red. The optimum doping concentration of
activator Sm3þ in YPO4 matrix is 2%. When the doping concentration of Sm3þ is higher than 2%, con-
centration quenching occurs. The mechanism of concentration quenching is electric dipole - electric
dipole interaction. The critical distance for energy transfer between Sm3þ is 1.899 nm. Nano-phosphor
Thermal characteristic
YPO4:2% Sm3þ has good thermal stability, and its fluorescence lifetime is 217.99
ms. The activation en-
ergy of thermal quenching is 0.2566 eV.
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
metastasis and tracer of cancer cells, and simultaneous multi-color
and multi-component labeling and detection of biomolecules [4,5].
Sm3þion has rich and strong light absorption properties in the near
ultraviolet - blue light region, and can excite orange-red fluores-
cence, which can achieve richer red luminescence than Eu3þ. Its
light absorption range matches the near-ultraviolet light emitted by
the InGaN chip [6e9].
The rare earth doped nano-phosphate luminescent materials
have been studied in many ways. Lai [10] synthesized Eu3þ doped
LaPO4 nano-crystalline by hydrothermal method, and studied ef-
fects of different crystal shapes on luminescence properties. Li [11]
studied surface synthesis, luminescence property and energy
transfer of LaPO4:Eu3þ,Sm3þnanorods. Angiuli [12] studied the
emission properties of YPO4:Sm3þ/Bi3þ phosphor. Wang [13]
studied the energy conversion between Sm3þ and Eu3þ in
Due to their unique 4f sub-layer electronic structure, large
atomic magnetic moment, strong spin-orbit coupling, variable co-
ordination number and crystal structure, rare earth elements have
rich optical, electrical and magnetic properties [1e3]. Rare earth
doped nano-luminescent materials have potential application in
integrated optical system, high definition display, medical biolog-
ical analysis, biochip, fluorescence bioimaging, drug carrier,
* Corresponding author. College of Materials and Metallurgy, Inner Mongolia
University of Science and Technology, Baotou, Inner Mongolia, 014010, PR China.
** Corresponding author. College of Materials and Metallurgy, Inner Mongolia
University of Science and Technology, Baotou, Inner Mongolia, 014010, PR China.
E-mail addresses: wujinxiu888@imust.cn (J. Wu), limei@163.com (M. Li).
https://doi.org/10.1016/j.jallcom.2019.153535
0925-8388/© 2019 Elsevier B.V. All rights reserved.

2
J. Wu et al. / Journal of Alloys and Compounds 821 (2020) 153535
Sr3La(PO4)3:Sm3þ,Eu3þ phosphors. Vineet [14] studied lumines-
cence properties of GdPO4:Er3þ,Yb3þ,Eu3þ phosphors excited by
ultraviolet and infrared light. Li [15] studied phase control of Eu3þ
doped YPO4 nano/microcrystal. Shao [16] synthesized YPO4:Ln3þ
(Ln ¼ Ce, Eu, Tb) nanocrystals, and studied their crystal structure
and multicolor luminescence properties. Deyneko [17] synthesized
Ca8MgSm1-х(PO4)7:xEu3þ red phosphor by solid-phase method.
Devi [18] synthesized Sm3þ doped yttrium orthophosphate nano-
particles with porous structure by simple coprecipitation method.
In the above studies, the luminescent properties of these phosphors
can be improved by changing the proportion, types of doped rare
earth elements and sensitizers. In this paper, using YPO4 as matrix,
a series of spherical nano-phosphors YPO4:Sm3þ were synthesized
by hydrothermal method. The morphology formation mechanism
of YPO4:Sm3þnanosphere was studied mainly, and the influence of
the structure and morphology of phosphors on the luminescent
properties was explored. The doping concentration of Sm3þ in
YPO4:Sm3þ phosphor, concentration quenching mechanism, fluo-
rescence lifetime and thermal stability were also studied.
was 370 nm ms pulsed flash lamp. All the samples were tested at
room temperature. In addition, the temperature dependent emis-
sion spectra of samples were measured using Hitachi F-4600
fluorescence spectrophotometer (Japan), and the temperature was
adjusted by Janis VPF-800 temperature controlling system.
3. Result and analysis
3.1. Study on formation mechanism of YPO4:Sm3þ morphology
The purpose is to study the formation mechanism of nano-
phosphor YPO4:Sm3þ morphology. Taking YPO4:2%Sm3þ as an
example, under the condition that the hydrothermal temperature is
200 ꢀC, the pH value is 3, and the moleratio of cation to anion is 1:3,
a series of YPO4:2%Sm3þ phosphors were synthesized with reaction
time of 0 h, 2 h, 4 h, 6 h, 8 h and 12 h. The phase structure and
morphology of YPO4:2%Sm3þ phosphor were studied, and the for-
mation mechanism of YPO4:2%Sm3þ morphology was analyzed.
2. Experimental techniques
3.1.1. XRD analysis
2.1. Materials and sample preparation
Fig. 1 shows XRD pattern of YPO4:2%Sm3þ prepared at different
reaction time. As shown in Fig. 1, the diffraction peak of the product
at 0 h does not correspond to the diffraction peak of the standard
card, indicating that the product at 0 h does not form crystals.
The positions of diffraction peaks of products with reaction time
from 2 h to 12 h are basically the same, which are in good agree-
ment with standard card JCPDS No: 84e0335 spectra of tetragonal
yttrium phosphate. There are no other impurity peaks, which in-
dicates that the products are single YPO4 crystals, and
Sm3þcompletely enters the lattice without causing any impurity. As
the reaction time increases from 2 h to 12 h, diffraction peak in-
tensity of the three strong peaks at (200), (112) and (312) crystal
planes gradually increases in the XRD diffraction pattern of the
products. The results indicate that the crystallinity of the product
becomes higher with the extension of reaction time. At 12 h reac-
tion time, the intensity of the diffraction peak are the strongest and
the shape of peak is much sharper, which indicates that the sample
has the highest crystallinity and the crystal growth is more com-
plete. It can be seen from the above analysis that a single tetragonal
nano-phosphor YPO4:2% Sm3þ can be synthesized at the hydro-
thermal temperature of 200 ꢀC, pH value of 3 and hydrothermal
reaction for 2 h. Nanophosphor YPO4:2%Sm3þ has the best crys-
tallinity after 12 h’s reaction.
A series of YPO4:Sm3þ(the Sm3þmolar concentration of 1%, 2%,
3%, 4%, 5% and 6%)nano-phosphors were synthesized by hydro-
thermal method. Firstly, Y2O3 and Sm2O3 (purity> 99.999%, Baotou
Research Institute of Rare Earths, China) were dissolved in 6 mol/L
HNO3 solution, respectively, to prepare 3 mol/L Y(NO3)3 and
0.3 mol/L Sm(NO3)3 solutions. According to the stoichiometric ratio
required for synthesis, a certain amount of Y(NO3)3 and Sm(NO3)3
solutions were weighed in a 150 mL beaker by pipette, and mixed
evenly to get solution A. A certain amount of concentrated phos-
phoric acid was dissolved in water to obtain 6 mol/L phosphoric
acid solution B. Then solution B was added to solution A drop by
drop under magnetic agitation, and the pH value of the reaction
system was adjusted to 3 with 3 mol/L NaOH solution. After stirring
for 10 min under the magnetic heating agitator, the beaker was
placed in the ultrasonic cleaner for 10 min. The reaction solution
was transferred into a 100 mL high-pressure reactor lined with
polytetrafluoroethylene, and the filling degree was 80%. The reactor
was put into the air drying box. The hydrothermal temperature is
200 ꢀC, the hydrothermal time is 0 h,2 h,4 h,6 h,8 h and 12 h
respectively, and the mole ratio of cations and anions is 1:3. The
final product was obtained after centrifugation, washing and drying
at room temperature, then ground with agate mortar, sealed and
tested.
2.2. Characterization
Phase analysis of the samples was carried out by the German D8
ADVANCE X-ray powder diffractometer with Cu K
a
(
l
¼ 1.5406 Å),
collection angle of 10ꢀe80ꢀ and scanning speeds of 4ꢀ/min and
0.02ꢀ/min. The crystal size, morphology, particle size, its distribu-
tion and composition elements of the product were determined by
the Sigma500 AMCS scanning electron microscope and the H-800
transmission electron microscopy. Infrared spectrum was
measured by Fourier transform infrared spectrometer (BROKER,
USA) using KBr tableting method. UVeVis absorption spectra of
samples were measured by U-3900 UVeViseNIR Spectrophotom-
eter (universal accessory with clamped integrating sphere) made
by Japan. The excitation and emission spectra of samples were
obtained with a Hitachi F-4600 fluorescence spectrophotometer
(Japan). Fluorescence lifetime was measured by FL920 series
steady/transient fluorescence spectrometer. The excitation source
Fig. 1. XRD pattern of YPO4:2%Sm3þ prepared at different reaction time.

J. Wu et al. / Journal of Alloys and Compounds 821 (2020) 153535
3
3.1.2. SEM analysis
YPO4:2%Sm3þwas first formed. With the extension of hydrothermal
reaction time, amorphous YPO4:2%Sm3þ gradually dissolved and
recrystallized to form YPO4:2%Sm3þ crystal nucleus. It was then
transformed into nano-spheres with high surface free energy. With
the further extension of the reaction time, the nano-spheres grew
gradually to further reduce the surface free energy. According to the
Ostwald ripening mechanism, to further reduce the surface free
energy, large particles gradually grew at the expense of consuming
small particles. From the analysis results of XRD and SEM, it can be
inferred that the formation of YPO4:2%Sm3þ crystal generally goes
through the following four processes: nucleation - dissolution -
recrystallization - crystal growth [20e22].
Fig. 2 shows field emission scanning electron microscope im-
ages of YPO4:2%Sm3þwith different reaction time. As shown in
Fig. 2, the product was loose and massive at 0 h, indicating that the
product did not form crystals. When the reaction lasted for 2 h, the
product appeared as spherical particles with uneven distribution
and poor dispersion. The particle size ranged from 30 to 70 nm. The
product also showed spherical particles when it reacted for 4 h. The
size of the particle was slightly larger (about 40e75 nm). When the
reaction time is 6 h, the particle diameter of the product was still
growing, and the particle size was about 50e80 nm. When the
reaction time was prolonged to 8 h, the particle diameter of the
product was about 60e90 nm, the distribution of the particles was
more uniform and the dispersion was better. When the reaction
time was 12 h, the product remained in spherical shape, the par-
ticles were dense and dispersed evenly, with the size of about
70e90 nm. It can be seen from the above analysis that with the
extension of reaction time, the spherical particles of the product
grew gradually, the dispersity became more and more uniform, the
compactness became higher and higher, and the crystallization
became more and more complete, which were in consistence with
the results of XRD analysis.
Fig. 3 shows the image of TEM, HRTEM and SAED of the product
after reacting 12 h.It is known that the product remained in
spherical shape, the particles were dense and dispersed evenly, and
the size was about 70e90 nm, which were consistent with the
results of SEM analysis. It can be seen from the HRTEM diagram that
the (200) interplanar spacing of the product was 0.3562 nm, which
was slightly greater than the (200) interplanar spacing of the
standard card JCPDS card 84e0335 by 0.3441 nm. It can be seen
from the selected area of the SAED diagram that crystallinity of the
product was high, which was consistent with the result of XRD
analysis. The crystal face index and the crystal zone axle of the
electron diffraction pattern is marked in Fig. 3.
3.1.3. Fluorescence spectrum analysis
Fig. 4b shows the emission spectra of phosphor YPO4:2%Sm3þ at
different reaction time in the excitation wavelength of 404 nm. The
fluorescence spectrum of Sm3þ ions is the result of transition from
6
5G5/2 energy level to the lower HJ energy level. Green emission at
564 nm, orange red emission at 603 nm and red emission at 648 nm
5
5
correspond to G5/2
/ , / /
6H5/2 5G5/2 6H7/2 and G5/2 6H9/2
transitions, respectively [23e25]. Fig. 4a shows the excitation
spectrum with the monitoring wavelength of 603 nm. There are
363, 367 and 390 nm wide peaks in the ultraviolet region, and 404,
418 and 440 nm wide peaks in the visible region. All of them are fef
transition in 4f configuration of Sm3þ. Their energy level transitions
were marked in Fig. 4 [25e28]. As shown in Fig. 4, the fluorescence
intensity of excitation spectrum and emission spectrum both in-
creases with the increase of reaction time. The product at 0 h has
weak luminescence intensity. The luminescence intensity of the
product at 0e2 h increased. The luminescence intensity of the
product increased sharply at 4e6 h and increased slowly at 6e12 h.
The luminescence intensity of the product at 12 h reaches its
maximum.
Based on the above analysis results; the nano phosphor with 12 h
reaction has the best crystallinity. The particle is dense and evenly
distributed. The particle size of the spherical nano fluorescent
powder is about 70e90 nm, and its luminous intensity is the largest.
It can be seen that the better the crystallinity and distribution of
YPO4:2%:Sm3þ phosphor, the better the fluorescence performance,
which is in consistence with the report in literature [22].
The microcrystalline morphology mainly attributes to the
combined action of kinetics and thermodynamics. The grain size
usually depends on competition between nucleation and growth
rate. The morphology evolution tends to decrease the surface free
energy [19]. When H3PO4 was added to the mixed solution of
yttrium nitrate and samarium nitrate, the amorphous precursor
Fig. 2. SEM of YPO4:2%Sm3þwith different reaction time.

4
J. Wu et al. / Journal of Alloys and Compounds 821 (2020) 153535
Fig. 3. Image of TEM, HRTEM and SAED of YPO4:2%Sm3þ
.
Fig. 4. Excitation (a) and emission spectrum (b) of YPO4:2%Sm3þ at different reaction time.
3.2. Study on luminescence property of YPO4:Sm3þ
samples are all yttrium phosphate with tetragonal structure, space
lattice of I41/amd(141), a ¼ b ¼ 6.882 Å, c ¼ 6.018 Å, Z ¼ 4, and
V ¼ 285.024 Å3. The strongest peak is located on the (200) crystal
plane. It can be seen from Fig. 4b that the crystal plane angle of
YPO4:Sm3þ diffraction peak (200) has changed slightly, and moved
slightly to a larger angle. This is because the 8 coordination ion
radius of samarium ion (0.096 nm) is larger than that of yttrium ion
(0.09 nm). During the formation of YPO4:Sm3þcrystals, when
samarium ions replace yttrium ions into the lattice, the lattice size
of the yttrium phosphate matrix increases slightly, and the crystal
structure is slightly distorted [29]. To further prove this problem,
the XRD patterns of YPO4:2%Sm3þ was refined, and the specific
results are shown in Fig. 6.
The effect of Sm3þ concentration (1%, 2%, 3%, 4%, 5% and 6%) on
spherical nano-phosphor YPO4: Sm3þ was studied under the con-
dition of 200 ꢀC, the mole ratio of cations and anions of 1:3, and
reaction time of 12 h, and the pH value of 3.
3.2.1. XRD analysis
Fig. 5 shows the XRD patterns of YPO4:Sm3þnanophosphors
with different Sm3þmole concentration. As shown in Fig. 5, XRD-
diffraction peak positions of YPO4 XRD-diffraction pattern and
phosphor YPO4:Sm3þ with different Sm3þ amounts are identical.
They are in perfect agreement with the diffraction peak positions of
standard card JPCDS No.084e0335, and there are no other
miscellaneous peaks. It indicates that the prepared samples are
pure phase in the purity range screened by XRD, the doping of Sm3þ
did not change the crystal structure of matrix YPO4. The prepared
The X-ray diffraction spectra of phosphor YPO4:2%Sm3þ were
refined by Rietveld method at room temperature, and the crystal
structure and lattice parameters of YPO4:2%Sm3þ were studied. The
results are shown in Figs. 6 and 7. The initial structure model of

J. Wu et al. / Journal of Alloys and Compounds 821 (2020) 153535
5
Fig. 5. XRD patterns of YPO4:Sm3þnano phosphors with different Sm3þ mole concentration.
YPO4:Sm3þ is constructed using I41/amd (141) space group [29,30].
As shown in Fig. 6, the XRD obtained by refining is in good agree-
ment with the tested XRD. The refined reliability parameters are
Rwp ¼ 14.36%, Rp ¼ 10.59%, Rexp ¼ 13.99% and GOF ¼ 1.03, which
are all within the acceptable range. The lattice parameters are
refined to a ¼ 6.889 Å, c ¼ 5.9902 Å, and lattice volume
V ¼ 285.794 Å3. Compared with the lattice parameters in standard
card JPCDS PDS No.084e0335, a slightly increases, c slightly de-
creases, and the lattice volume slightly increases. It can be seen that
when samarium ions replace yttrium ions into the lattice, the lat-
tice size of the yttrium phosphate matrix changes slightly, causing a
slight distortion in the crystal structure. This is consistent with the
results reported in literature [29,30].
3.2.2. Analysis of SEM and EDS
Fig.
8 is the scanning electron microscopic image of
YPO4:Sm3þnanophosphors with 2% and 6% Sm3þ molar content. As
shown in Fig. 8, the morphology of products are all spherical par-
ticles with uniform size distribution and good dispersion. When
Sm3þ content is 2%, the spherical particle size is about 50e60 nm.
When Sm3þ concentration is 6%, the spherical particle size is about
60e70 nm. It can be seen that the particle size of nano-particles
increased slightly with the increase of the Sm3þ mole content,
which may be that the radius of Sm3þ (0.096 nm) is larger than that
of Y3þ (0.090 nm). When the mole concentration of Sm3þ increases,
the number of Sm3þ replacing Y3þ will increase accordingly, so the
particle diameter of the final nano-spherical product increased
slightly.
Fig. 6. The Rietveld refinement of XRD pattern of YPO4:2%Sm3þ powder.
From the EDS in Fig. 9, it is known that components of YPO4:2%
Sm3þnano-phosphor include P, O, Y and Sm. Quantitative analysis
shows that the atomic number ratio of each element is Sm þ Y/P/
O ¼ 1/1/4.7, which is basically matches that (Y/P/O ¼ 1/1/4) of the
yttrium phosphor YPO4 with tetragonal structure. No peaks of
other elements were found, indicating that the prepared sample is
yttrium phosphate in pure phase. This is consistent with the result
of XRD analysis.
3.2.3. Infrared spectrum analysis
Fig. 10 shows infrared spectrum of phosphor YPO4:2%Sm3þ
,
from which the infrared absorption bands of each interval can be
observed. In Fig. 10, the wide absorption band at 1119e984 cmꢁ1 is
the antisymmetric stretching vibration of the PeOeP bond. The
absorption band at 650 cmꢁ1 is the antisymmetric bending vibra-
tion of PeO [31,32]. The absorption band at 500 cmꢁ1 is the vi-
bration of YeO and SmeO. The two absorption bands at 3486 and
Fig. 7. Schematic crystal structure of YPO4:2%Sm3þphosphor.

6
J. Wu et al. / Journal of Alloys and Compounds 821 (2020) 153535
Fig. 8. SEM patterns of nanophosphorsYPO4:Sm3þ with different Sm3þ mole concentration.
1655 cmꢁ1 belong to OeH bending and stretching vibration ab-
sorption peaks in water, indicating that water is absorbed on the
product surface [33].There is a weak absorption band between
2402 and 2275 cmꢁ1, which may be that the product absorbed
carbon dioxide from the air during the test.
3.2.4. Ultraviolet absorption spectrum and energy gap
As shown in Fig. 11, there is a wide absorption peak in
200e250 nm ultraviolet region, which is the absorption peak of the
matrix. There are a series of absorption peaks in 300e500 nm,
where the peak at 404 nm is relatively obvious. It is the charac-
6
teristic absorption peak of Sm3þ, which attributes to the H5/2
/
4F7/2 transition. According to literature [33e35], YPO4:2%Sm3þ is a
direct band-gap semiconductor. The energy gap Eg can be obtained
by the following equation.
ꢀ
ꢁ
ð
a
hvÞ2 ¼ A hv ꢁ Eg
(1)
Fig. 9. EDS of nano-phosphor YPO4:2%Sm3þ
.
Where,
a
represents absorption coefficient, h represents Planck
constant,
n
represents incident light frequency and A represents
constant. As shown in Fig. 10, energy gap (Eg) of YPO4:2%Sm3þ is
5.21eV, which is similar to the optical band-gap reported in liter-
ature [31].
3.2.5. Excitation and emission spectrum and chromaticity
coordinates
As shown in Fig. 12a, when the monitoring wavelength is
603 nm, the excitation spectrum of samples are composed of a
series of narrow-band excitation peaks between 350 and 500. They
are the fef transition absorption of 4f configuration of Sm3þ, where
the strongest absorption occurred at 404 nm. They belong to the
transition absorption peaks of Sm3þat 363 nm (6H5/2
/
4D3/2),
4
4
376 nm (6H5/2 / D1/2), 404 nm (6H5/2 / F7/2), 418 nm (6H5/2
/
4
6P5/2), 441 nm (6H5/2 / 4G9/2 þ I15/2), 466 nm (6H5/2 / 4I13/2) and
478 nm (6H5/2 / 4I11/2), respectively [35,36]. It can be seen that the
nano-phosphor YPO4:Sm3þprepared in this paper can be effectively
Fig. 10. FT-IR of nano-phosphor YPO4:2%Sm3þ
.

J. Wu et al. / Journal of Alloys and Compounds 821 (2020) 153535
7
Fig. 13, chromaticity coordinates of the optimum product YPO4:2%
Sm3þ are X ¼ 0.5930, Y ¼ 0.4060. Chromaticity coordinates of
YPO4:3%Sm3þ are X ¼ 0.5902, Y ¼ 0.4091. It can be seen that the
phosphors are orange red.
3.2.6. Critical distance for energy transfer and mechanism of
concentration quenching
Concentration quenching refers that the emission intensity of
phosphors begins to decrease when the concentration of the doped
activator exceeds a certain value in the matrix. The activator con-
centration at this point is the luminescence quenching concentra-
tion of this fluorescence material, also known as the critical
concentration [39]. When concentration quenching occurs, the
critical distance between activator and matrix ion can be calculated.
According to the study of Blasse [40], the critical distance of energy
transfer can be calculated as follows:
ꢂ
ꢃ
1=3
3V
CN
Rc ¼ 2 ꢂ
(2)
Fig. 11. Relationship between Ahn2 and h of nano-phosphor YPO4:2%Sm3þ
n .
4
p
V represents unit cell volume, N represents coordination num-
ber, and C represents quenching concentration. For the YPO4:2%
Sm3þ crystal in this experiment, N ¼ 4, V ¼ 0.286 nm3, and the
quenching concentration of Sm3þ is 0.02 mol/L. By substituting
them into equation (2), Rc ¼ 1.899 nm can be obtained, i.e., the
critical distance of YPO4:2%Sm3þ energy transfer is 1.899 nm. It can
be seen that the critical distance is much larger than 0.5 nm, so the
energy transfer mechanism between Sm3þ is the interaction be-
tween multiple electric dipoles.
excited by near ultraviolet and blue LED. As shown in Fig.12b, in the
case of 404 nm visible-light excitation, emission peaks of
YPO4:Sm3þ at 564 nm, 603 nm, 648 nm and 708 nm are the main
characteristic emission peaks of Sm3þ. Fluorescence spectrum of
5
Sm3þ ion is the transition from G5/2 energy level to the lower 6HJ
energy level, which respectively belong to 4G5/2/6HJ(J ¼ 5/2,7/2,9/
2,11/2) transitions [36,37], and the luminescence intensity reaches
its maximum at 603 nm.
In order to further analyze the mechanism of energy transfer,
according to Dexter’s theory of energy transfer [41,42], the rela-
tionship between luminescence intensity I and activator concen-
tration X is as follows:
As shown in Fig. 12, with the increase of the doping concen-
tration of Sm3þ, the peak positions of the excitation spectrum and
emission spectrum of nano-phosphor YPO4:Sm3þwere completely
consistent, however, the fluorescence intensity first increased and
then decreased. When the doping concentration of Sm3þ reaches
2%, the intensity of excitation spectrum and emission spectrum
reached the maximum. Therefore, the optimum doping molar
concentration of Sm3þ in YPO4 matrix is 2%. When the doping
molar concentration of Sm3þ is more than 2%, concentration
quenching occurs.
Emitting color is also very important for the practical applica-
tion of phosphors [38]. In this experiment, we used CIE 1931 soft-
ware to calculate chromaticity coordinates of the nano-phosphor
YPO4:Sm3þ excited by 404 nm near-ultraviolet light and drew the
chromaticity coordinates diagram of CIE 1931 in Fig.13. As shown in
.
ꢄ
ꢅꢁ1
I
X∞
b
XS=3
or lgðI = XÞ ¼ C ꢁ ðS = 3ÞlgX
(3)
b
is a constant, S is the exponent of multielectrode interaction.
With logX as abscissa, and lg(I/X) as ordinate, a straight line with
the slope of -S/3 is obtained. If S ¼ 3, it is the reciprocal interchange.
When S ¼ 6, it is electric dipole - electric dipole interaction. When
S ¼ 8, it is electric dipole - electric quadrupole interaction. When
S ¼ 10, it is electric quadrupole - electric quadrupole interaction.
Fig. 14 shows the relationship between lg(I/X) and lgx when
Sm3þ doping concentration is more than the optimum
Fig. 12. Excitation spectrum (b) and emission spectrum (a) of nano-phosphor YPO4:2%Sm3þ
.

8
J. Wu et al. / Journal of Alloys and Compounds 821 (2020) 153535
Fig. 15. Life time decay curve of nano-phosphor YPO4:Sm3þ with different Sm3þ
concentration.
transition of YPO4:Sm3þphosphors. Their excitation wavelengths
are all 370 nm, and monitoring wavelengths are 603 nm. The
experimental data conform to the second order exponential decay
function (4) [43,44]:
IðtÞ ¼ I0 þ A1 expð ꢁ t = t1Þ þ A2 expð ꢁ t = t2
Þ
(4)
Fig. 13. Chromaticity coordinates of nano-phosphor YPO4:Sm3þ
.
where, I(t) and I0 are the emission intensities at time t and 0,
respectively; A1 and A2 are the fitting constants; t1 and t2 are the
fast and slow components of the decay time. The average lifetime
(tav) can be evaluated by the following equation (5) [44,45]:
ꢄ
ꢅ.
tav ¼ A1t21 þ A1t22 ðA1t1 þ A1t2
Þ
(5)
The average life of YPO4:Sm3þ phosphor is calculated. The result
is shown in Fig. 15, in which the fluorescence lifetime of YPO4:2%
Sm3þ is the maximum (217.99
ms). With the increase of the doping
concentration of Sm3þ, the fluorescence lifetime of the phosphor
first increases and then decreases. This change rule is consistent
with that of excitation and emission spectra in Fig. 12.
3.3. Fluorescence temperature characteristic of nano-phosphor
YPO4:2%Sm3þ
The thermal stability of phosphor is very important for its
practical application, especially in the application of high power
LED, it is a key technical problem. Fig. 16a shows the high tem-
perature fluorescence spectra of nano-phosphor YPO4:2%Sm3þin
the range of 300e530 K with excitation wavelength of 404 nm.As
shown in Fig. 16a, as the temperature rises, the shape and peak
position of emission spectrum do not change, however, the fluo-
rescence intensity decreases with the increase of temperature. This
is caused by the thermal quenching effect [46].
As shown in Fig. 16b, when the temperature goes up to 406 K,
the luminescence intensity of YPO4:2%Sm3þ phosphor is 87.6% of
that at room temperature. When the temperature rises further to
480 K, the luminescence intensity can still maintain 74.8% of that at
room temperature. When the temperature continues to rise to
530 K, the luminescence intensity can also maintain 74.4% of that at
room temperature. When the temperature is above 480 K, the
luminescence intensity of nano-phosphor YPO4:2%Sm3þ decreases
slowly, which indicates that this kind of nano-phosphor YPO4:2%
Sm3þ has good thermal stability, and has potential as a red phos-
phor for white LED excited by visible and near-ultraviolet light.
The relationship between relative emission intensity and tem-
perature can be expressed by equation (6) [47,48], by which
Fig. 14. The relationship between lg(I/X) and lgx of nano-phosph or YPO4:Sm3þ
.
concentration (X ¼ 2.0%), and I is the intensity of the emission peak
at 603 nm Origin8.5 software was used for linear fitting of exper-
imental points in Fig. 12b, and a straight line was obtained. lg(I/X)
and lgX are basically linear (correlation coefficient is 0.9522), with a
slope of ꢁ1.921. From this slope, the S value can be calculated to be
about 6. Therefore, the mechanism of concentration quenching
between Sm3þ is the electric dipole - electric dipole interaction.
3.2.7. Fluorescence lifetime
Fluorescence lifetime (Decay lifetime) is one of the important
parameters to study the luminescence properties of phosphors.
When the sample is excited with laser, the electron of the sample
will transit from the ground state to an excited state, then radiate,
emit light and return to the ground state. Fluorescence lifetime
refers to the time required for the number of ions in the excited
state to decrease to 1/e when the excitation stops.
4
Fig. 15 shows the fluorescence decay curves of G5/2
/
6H7/2

J. Wu et al. / Journal of Alloys and Compounds 821 (2020) 153535
9
Fig. 16. (a)Fluorescence spectra of YPO4:2%Sm3þ at different temperatures, (b)The relationship between the luminescence intensity of nanophosphor YPO4:2%Sm3þ and (c)The
change of Ln[(I0/I)-1] and 1/KT activation energy.
thermal quenching activation energy of phosphor YPO4:2%Sm3þ
can be calculated.
design of the experimental scheme.
Jinxiu Wu and Huiling Jia mainly completed the analysis of
experimental data, and writing and revision of the paper, etc.
Jinxiu Wu and Hengjun Jia mainly completed the synthesis and
characterization of nano materials, and the operation of in-
struments, etc.
ꢂ
ꢆ
ꢇꢃꢁ1
D
KT
E
IT ¼ I0 1 þ C exp ꢁ
(6)
Where, I0 is the initial fluorescence intensity, IT refers to fluores-
cence intensity at different temperatures, E refers to quenching
Zhongzhi Wang completed the characterization experiments of
some materials.
D
activation energy, C is a constant, K is boltzmann constant
(8.62 ꢂ 10ꢁ5eV) [47,48], and T refers to temperature. The experi-
mental data in Fig. 16b were processed and fitted to obtain the
straight line in Fig. 16c with the slope of ꢁ0.2555, which indicates
that thermal quenching activation energy of nano-phosphor
YPO4:2%Sm3þ is 0.2566 eV.
Declaration of competing interest
We declare that we do not have any commercial or associative
interest that represents a conflict of interest in connection with the
work submitted.
4. Conclusions
Acknowledgments
The nano-phosphor YPO4:Sm3þ synthesized by hydrothermal
method is a single tetrahedral phosphor yttrium ore structure.
With the increase of reaction time, the crystallinity and fluores-
cence properties of nano-phosphor increase gradually. After the
reaction time is 12 h, the morphology is nano-sphere, and the
particle size is about 30e90 nm. Morphology formation mechanism
of YPO4:Sm3þ is nucleation - dissolution - recrystallization - crystal
growth. Spherical nano-phosphor YPO4:Sm3þ can be excited by
404 nm visible light, and has the strong red emission of 603 nm.
The fluorescence lifetime of nano-phosphor YPO4:Sm3þ is between
This work was supported by a Grant from fund of Nature Science
of China (Grant No.51965053, 51634005and 51662033), and fund of
Nature Science of Inner Mongolia autonomous region, China (No.
2017MS(LH)0208, 2018MS02016).
References
[1] X. Huang, B. Li, H. Guo, D. Chen, Molybdenum-doping-induced photo-
luminescence enhancement in Eu3þ -activated CaWO4 red-emitting phos-
phors for white light-emitting diodes, Dyes Pigments (2017) 86e94.
[2] R.R. Dasilva, M. Cavicchioli, L.R. Lima, C.G. Otoni, H.S. Barud, S.H. Santagneli,
S.J. Ribeiro, Fabrication of biocompatible, functional, and transparent hybrid
films based on silk fibroin and epoxy silane for biophotonics, ACS Appl. Mater.
Interfaces 9 (33) (2017) 27905e27917.
[3] F. Wang, X. Liu, Recent advances in the chemistry of lanthanide-doped up
conversion nanocrystals, Chem. Soc. Rev. 38 (2009) 976e989.
[4] S. Rodriguez-Liviano, A.I. Becerro, D. Alcaantara, V. Grazu, J.M.D. Fuente,
M. Ocana, Synthesis and properties of multifunctional tetragonal Eu:GdPO4
nanocubes for optical and magnetic resonance imaging applications, Inorg.
Chem. 52 (2013) 647e654.
[5] C.D. Walkey, W.C.W. Chan, Understanding and controlling the interaction of
nanomaterials with proteinsin a physiological nvironment, Chem. Soc. Rev. 41
(2012) 2780e2799.
[6] H.X. Jiang, S.C. Lün, Intense red emission and two-way energy transfer in
Sm3þ, Eu3þco-doped NaLa(WO4)2 phosphors, Mater. Res. Bull. 11 (2019),
1470e145.
202.33 and 217.99ms. With the increase of the doping concentration
of activator Sm3þ, intensity of the excitation and emission spectra
and fluorescence lifetime of nano-phosphors first increase and then
decrease. The optimum doping concentration of activator Sm3þ is
2%. When the doping concentration of Sm3þ is greater than 2%,
concentration quenching appears. Mechanism of concentration
quenching of Sm3þ is interaction between electric dipoles. The
critical distance of YPO4:Sm3þenergy transfer is 1.899 nm YPO4:2%
Sm3þ nano-phosphor has good thermal stability, and its thermal
quenching activation energy is 0.2566 eV. The optimum product
YPO4:2%Sm3þ in this paper has potential application prospects in
the field of white LED excited by near-ultraviolet light chip, and can
be used as the backup material of red phosphor.
[7] X. Zhang, G. Zhou, J. Zhou, H. Zhou, P. Kong, Z. Yu, J. Zhan, Fabrication and
enhanced photoluminescence properties of NaLa(MoO4)2:Sm3þ,Bi3þ phos-
phors with high efficiency white-light-emitting, Bull. Mater. Sci. 40 (5) (2017)
983e990.
Author contributions section
[8] H. Wu, J. Yang, X. Wang, S. Gan, L. Li, Tune color of single-phase LiGd (MoO4)2-
(WO4) X: Sm3þ, Tb3þ via adjusting the proportion of matrix and energy
X
Jinxiu Wu, Mei Li and Zhaogang Liu mainly completed the
transfer to create white-light phosphor, Solid State Sci. 77 (2018) 20e26.

10
J. Wu et al. / Journal of Alloys and Compounds 821 (2020) 153535
[9] J.X. Wu, M. Li, Z.G. Liu, Y.H. Hu, M.T. Wang, Controllable morphology and
J. Photochem. Photobiol. A Chem. 353 (2018) 65e70.
luminescence properties of SrMoO4:Sm3þ,Naþ red emitting phosphors, Chin. J.
Inorg. Chem. 33 (2) (2017) 219e226.
[28] D. Hou, X. Pan, J. Li, W. Zhou, X. Ye, Structure and luminescence properties of
Sm3þ doped Y2MoO6 phosphor under near ultraviolet light excitation, J. Rare
Earths Chin. 35 (4) (2017) 335e340.
[10] H. Lai, A. Bao, Y.M. Yang, Y. Tao, H. Yang, Selective synthesis and luminescence
property of monazite and hexagonal-type LaPO4:Eunanocrystals, Crys-
tEngComm 11 (2009) 1109e1113.
[29] H. Guo, B. Devakumar, B. Li, X. Huang, Novel Na3Sc2(PO4)3:Ce3þ,Tb3þ phos-
phors for white LEDs: tunable blue-green color emission, high quantum ef-
ficiency and excellent thermal stability, Dyes Pigments 151 (2018) 81e88.
[30] L. Li, X. Tang, Z. Jiang, X. Zhou, S. Jiang, X. Luo, G. Xiang, K. Zhou, NaBa-
La2(PO4)3:A novel host lattice for Sm3þ-doped phosphor materials emitting
reddish-orange light, J. Alloy. Comp. 701 (2017) 515e523.
[11] G. Li, L. Li, M. Li, Y. Song, H. Zou, L. Zou, S. Gan, Facile synthesis and lumi-
nescent properties of LaPO4:Eu3þ,Sm3þnanorods via
a designed two-step
hydrothermal method, Mater. Chem. Phys. 133 (2012) 263e268.
[12] F. Angiuli, E. Cavalli, P. Boutinaud, R. Mahiou, Emission properties of Sm3þ
Bi3þ-doped YPO4 phosphors, J. Lumin. 135 (2013) 239e242.
/
[31] J.X. Wu, M. Li, Z.G. Liu, M.T. Wang, H.L. Jia, Preparation and optical properties
of one-dimensional nanomaterial SmPO4$0.5H2O, J. Lumin. Chin. 10 (2018)
1395e1404.
[13] Z. Wang, S. Lua, P. Li, Luminescent properties and energytransfer of
Sr3La(PO4)3:Sm3þ,Eu3þfor white LEDs, J. Alloy. Comp. 586 (2014) 536e541.
[14] K. Vineet, S. Sukhvir, C. Santa, Fabrication of dual excitation, dual emission
nanophosphor with broad UV and IR excitation through simultaneous doping
of triple rare earth ions Er3þ ,Yb3þ, Eu3þ in GdPO4, Superlattice. Microst 79
(2015) 86e95.
[32] J. Wu, M. Li, M. Wang, Z. Liu, H. Jia, Preparation and luminescence properties
of NaLa(WO4)2:Sm3þ orange-red phosphor, J. Lumin. 197 (2018) 219e227.
[33] M. Yagoub, H. Swart, E. Coetsee, Energy transfer study between Ce3þ and Tb3þ
ions in a calcium fluoride crystal for solar cell applications, J. Lumin. 187
(2017) 96e101.
[15] P. Li, Y. Zhang, L. Zhang, F. Li, Y. Guo, Y. Li, W. Gao, Phase control of Eu3þ
-
doped YPO4 nano-/microcrystals, Cryst. Growth Des. 17 (11) (2017)
5935e5944.
[16] B. Shao, Y. Feng, S. Zhao, S. Yuan, J. Huo, Phase-tunable synthesis of mono-
disperse YPO4:Ln3þ(Ln ¼ Ce, Eu, Tb) micro/nanocrystals via topotactic trans-
formation route with multicolor luminescence properties, Inorg. Chem. 56
(11) (2017) 6114e6121.
[17] D.V. Deyneko, I.V. Nikiforov, B.I. Lazoryak, D.A. Spassky, I.I. Leonidov,
S.Y. Stefanovich, P.C. Burns, Ca8MgSm1-х(PO4)7:xEu3þ, promising red phos-
phors for WLED application, J. Alloy. Comp. 776 (2019) 897e903.
[18] F.D. Heikham, D.S. Thiyam, Optical properties of porous Sm3þ-doped yttrium
orthophosphate nanoparticles tailored by co-precipitation route, Opt. Com-
mun. 439 (2019) 34e37.
[19] R. Yu, H.M. Noh, B.K. Moon, B.C. Choi, J.H. Jeong, H.S. Lee, K. Jang, S.S. Yi,
Photoluminescence characteristics of Sm3þ doped Ba2CaWO6 as new orange-
red emitting phosphors, J. Lumin. 152 (2014) 133e137.
[20] Y. Zhai, X. Zhao, Q. Sun, S. Yang, D. Deng, Y. Liu, J. Wang, Template-free hy-
drothermal synthesis and luminescence properties of NaGd(WO4)2:Eu3þ, red
phosphors with controlled morphology, J. Mater. Sci. Mater. Electron. 29 (6)
(2018) 5034e5041.
[21] Z. Wang, J. Zhong, H. Jiang, J. Wang, H. Liang, Controllable synthesis of Na
Lu(WO4)2:Eu3þ microcrystal and luminescence properties for LEDs, Cryst.
Growth Des. 14 (8) (2014) 3767e3773.
[34] Maheshwary, B. Singh, R. Singh, Color tuning in thermally stable Sm3þ-acti-
vated CaWO4 Nanophosphors, New J. Chem. 39 (2015) 4494e4507.
[35] M. Li, J. Wu, H. Jia, M. Wang, Z. Liu, Luminescence properties and energy
transfers of NaLa(WO4)2:Sm3þ:Ce3þ phosphor, J. Mater. Sci. Mater. Electron.
30 (2019) 10465e10474.
[36] J.X. Wu, M. Li, Z.G. Liu, Y.H. Hu, M.T. Wang, Effects of doped concentration and
calcination temperature on luminescence properties of Ca1-xSmxWO4 phos-
phors, Chin. J. Inorg. Chem. 32 (1) (2016) 34e42.
[37] V.V. Atuchin, A.S. Aleksandrovsky, O.D. Chimitova, C.P. Diao, T.A. Gavrilova,
V.G. Kesler, Electronic structure of b-RbSm(MoO4)2 and chemical bond ingin
molybdates, Dalton Trans. 44 (2015) 1805e1815.
[38] C.S. McCamy, Correlated color temperature as an explicit function of chro-
maticity coordinates, Color Res. Appl. 17 (1992) 142e144.
[39] B. Devakumar, P. Halappa, C. Shivakumara, Dy3þ/Eu3þ co-doped CsGd(MoO 4
)
2
phosphor with tunable photoluminescence properties for near-UV WLEDs
applications, Dyes Pigments 137 (2017) 244e255.
[40] G. Blasse, Energy transfer in oxidic phosphors, Philips Res. Rep. 24 (1969)
131e144.
[41] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21
(1953) 836e850.
[42] M. Yan, G. Liu, J. Wen, Y. Wang, Blue-Light-Excited Eu3þ/Sm3þ Co-doped
NaLa(MoO4)2 phosphors: synthesis, characterizations and red emission
enhancement for WLEDs, Materials 11 (7) (2018) 1090e1091e11.
[43] N. Guo, Y.J. Huang, M. Yang, Y.H. Song, Y.H. Zheng, H.P. You, A tunable single
component warm white-light Sr3Y(PO4)3:Eu2þ, Mn2þ phosphor for white-
lightemitting diodes, Phys. Chem. Chem. Phys. 13 (2011) 15077e15082.
[44] L. Han, D.H. Xu, Q.G. Xu, Q.M. Di, J.Y. Sun, Synthesis and luminescence prop-
erties of Sr3GdNa(PO4)3F: Sm3þ phosphor, J. Mater. Sci. 50 (2015) 2257e2262.
[45] J.Y. Sun, Y.N. Sun, J.H. Zeng, H.Y. Du, Luminescence properties and energy
transfer investigations of Sr3Gd(PO4)3:Ce3þ,Tb3þ phosphors, J. Phys. Chem.
Solids 74 (2013) 1007e1011.
[22] J. Wu, M. Li, H. Jia, Z. Liu, M. Wang, Mechanism of morphology formation and
energy transfer of NaLa(WO4)2:Sm3þ:Eu3þ phosphors, Appl. Phys.
A 125
(2019) 438e452.
[23] G. Li, Y. Wei, W. Long, G. Xu, Photoluminescence properties, energy transfer
and thermal stability of the novel red-emit-ting CaGd2(WO4)4:Eu3þ, Sm3þ
phosphors, Mater. Res. Bull. 95 (2017) 86e94.
[24] Q. Yang, G. Li, Y. Wei, H. Chai, Synthesis and photoluminescence properties of
red-emitting NaLaMgWO6:Sm3þ,Eu3þ phosphors for white LED applications,
J. Lumin. 199 (2018) 323e330.
[25] H. Li, X. Gong, Y. Chen, J. Huang, Y. Lin, Z. Luo, Y. Huang, Luminescence
properties of phosphate phosphors Ba3Gd1-x(PO4)3:xSm3þ, J. Rare Earths Chin
36 (5) (2018) 456e460.
[46] B. Li, X. Huang, H. Guo, Y. Zeng, Energy transfer and tunable photo-
luminescence of LaBWO6:Tb3þ,Eu3þphosphors for near-UV white LEDs, Dyes
Pigments 150 (2018) 67e72.
[26] W. Xia, S. Xiao, X. Yang, X. Jin, Quantum cutting and tunable luminescence
properties in Pr3þ/Sm3þ,Yb3þ co-doped SrMoO4 powders, Mater. Res. Bull. 89
(2017) 5e10.
[47] P.L. Li, Z.J. Wang, Q.L. Guo, Z.P. Yang, X. Li, F. Teng, Mater. Res. Bull. 70 (2015)
789e794.
[48] P. Shi, Z. Xia, M.S. Molokeev, V.V. Atuchin, Crystal chemistry and luminescence
properties of red-emitting CsGd1ꢁxEux(MoO4)2 solid-solution phosphors,
Dalton Trans. 43 (25) (2014) 9669e9676.
[27] X. Tan, Y.R. Wang, M.Y. Zhang, Solvothermal synthesis, luminescence and
energy transfer of Dy3þ and Sm3þ doped NaLa(WO4)2 nanocubes,